Climate Outlook

NORTH AMERICA September 2002 - February 2003

Issued: Aug 2002

The IRI has prepared this experimental Climate Outlook for North America
for September 2002 - February 2003. Of relevance in the preparation of this
outlook is the prediction of warmer than average conditions in the central
and eastern equatorial Pacific for the next 6 to 9 months.
Currently the sea surface temperatures (SSTs) across much of
the central and eastern equatorial Pacific are more than 1 degree C above
their long-term average
(SSTs),
although slightly lower than average SSTs were observed in the far eastern Pacific,
near South America, during July. Weak warm SST anomalies (approximately 0.5 degree C)
are predicted in our evolving SST scenario over the central and
eastern equatorial Pacific throughout the forecast period,
September-November 2002,
October-December 2002,
November-January 2003,
December-February 2003.
However, the climate forecasts reflect an assumption that the magnitude of
SST anomalies in the central and eastern Pacific is more likely to remain
near that of the
recent observations.
Warmer than average SSTs currently observed over the eastern half of the
Indian Ocean are expected to migrate towards the central part of the basin,
with slightly stronger amplitude than that suggested by our statistical
prediction, which is included in the evolving SST scenario. The area of
above-average temperature in the equatorial Atlantic Ocean is
expected to persist through at least the first half of the forecast
period.

METHODS -

This Outlook was prepared using the following procedures and
information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific SST
covering the forecast period. Particularly heavy weighting has been
given to predictions from the coupled model operated by the NOAA
National Centers for Environmental Prediction, Climate Modeling Branch.
This model suggests a continuation of near-average conditions during the
first forecast season. The forecast for near-neutral conditions is
consistent with some, but not all, numerical and statistical forecasts
of central and eastern Pacific SSTs.

B) Forecasts of the tropical Indian ocean using a statistical model
developed by the IRI.

C) Global atmospheric general circulation model (GCM) predictions of the
atmospheric response to the present and predicted sea-surface
temperature patterns.

The procedures, models, and data used to derive this Climate Outlook may
be somewhat different from those used by the national meteorological
services in the region. Thus, this product may differ from the official
forecasts issued in those areas. The Climate Outlook for
September 2002 - February 2003
is dependent on the accuracy of the SST predictions. For the
tropical Pacific, these predictions can be expected to provide useful
information, but there is some uncertainty concerning the evolution of
SSTs. Spread in global SST predictions is a source of uncertainty in the
Outlook provided here. Note that even if perfectly accurate SST
forecasts were possible, there would still be uncertainty in the climate
forecast due to chaotic internal variability of the atmosphere. These
uncertainties are reflected in the probabilities given in the forecast.

It is stressed that the current status of seasonal-to-interannual
climate forecasting allows prediction of spatial and temporal averages,
and does not fully account for all factors that influence regional and
national climate variability. This Outlook is relevant only to seasonal
time scales and relatively large areas; local variations should be
expected, and variations within the 3-month season should also be
expected. For further information concerning this and other guidance
products, users are strongly advised to contact their National
Meteorological Services.

OUTLOOK -

This Outlook covers four seasons: September-November 2002,
October-December 2002, November-January 2003 and December-February 2003.
Maps are given showing tercile probabilities of
precipitation and temperature. The maps for precipitation indicate the
probabilities that the seasonal precipitation will fall into the wettest
third of the years (top number), the middle third of the years (middle
number), or the driest third of the years (bottom number). The color
shading indicates the probability of the most dominant tercile -- that
is, the tercile having the highest forecast probability. The color bar
alongside the map defines these dominant tercile probability levels. The
upper side of the color bar shows the colors used for increasingly
strong probabilities when the dominant tercile is the above-normal
tercile, while the lower side shows likewise for the below-normal
tercile. The gray color indicates an enhanced probability for the
near-normal tercile (nearly always limited to 40%). As before, numbers
and their associated histograms show the probabilities of the three
terciles. In areas with lots of spatial detail, there may not be
sufficient room on the map, to allow histograms for each region. In
those cases, some idea of the probabilities may be gained from the color
alone. A qualitative outlook of climatology ("C") indicates that there
is no basis for favoring any particular category.
Areas that are marked by "D" represent regions for which less than 3cm of
precipitation typically occurs over the season.
Otherwise, for example, in the case of
September-November 2002 (Map A), there
is a 25% probability that the precipitation will be in the wettest third
of the years, a 35% chance it will be in the near-normal third of the
years, and a 40% chance that the precipitation will be in the driest
third of the years in the northwestern U.S.

Maps of temperature show expected probabilities that the seasonal
temperatures will fall into the warmest third of the years, the middle
third of the years, or the coldest third of the years
(Map A).
The numbers for each region on the temperature maps
indicate the probabilities of temperatures to fall in each of the three
categories, above-, near-, and below-normal.

An
additional precipitation map
is provided for the first season indicating probabilities for extreme
precipitation anomalies. Extremes are defined as anomalies that fall
within the top and bottom 15th percentile of the observed records. A
priori, there is a 15% probability of being within the extremely wet
category, and a 15% probability of being within the extremely dry
category, leaving a 70% probability that the precipitation will not be
extreme. The maps indicate areas of increased risk of extreme
precipitation totals. Three levels of increased risk are defined:
slightly enhanced risk, enhanced risk, and greatly enhanced risk. For
slightly enhanced risk, there is a 25-40% probability that precipitation
will be within the indicated extreme, i.e. wet or dry. This represents
an approximate doubling of the climatological risk. For enhanced risk,
there is a 40-50% probability that precipitation will be within the
indicated extreme. This represents an approximate tripling of the
climatological risk. For greatly enhanced risk, the probability that
precipitation will be within the indicated extreme exceeds 50%, i.e. the
indicated extreme is the most likely outcome. A similar map is provided
in the first season indicating probabilities of
extreme temperature
anomalies.

Boundaries between sub-regions should be considered as transition zones,
and their location considered to be only qualitatively correct.

September-November 2002 through December-February 2003:

The following discussion briefly describes the probability anomaly forecasts:

Precipitation

Slightly enhanced probabilities for above normal precipitation are
forecast the southern coast of Alaska throughout the four forecast
periods, as well as across parts of the southern U.S. and northern
Mexico. Slightly enhanced probabilities for below normal precipitation are
forecast for the northwest U.S. and southwestern Canada for all four
periods, and for the region in and around the U.S. Great Lakes, in
part of the Caribbean, and in southern Central America for the third
and fourth forecast periods.

Temperature
Enhanced probabilities for above normal temperature are forecast
for all four forecast periods in parts of Canada, northern U.S., and
particularly in southern Mexico and central America.
Enhanced probabilities for near normal temperature are forecast
for part of the southeastern U.S. during the third and fourth
forecast periods.